A liquid crystal display (LCD) is generally used for a personal computer, a monitor of a mobile device and a television in view of several merits, such as low voltage, low electric power requirement, miniaturization and thinner shape. Usually, the liquid crystal display is constructed of the liquid crystal cell, an optical compensation sheet and a polarizer. In a technical field of the optical materials, various types of polymer film products are used for protecting a polarizing filter, reducing the coloring of an image and widening a view angle.
As the polymer film preferably used for the LCD, a cellulose acylate film has high birefringence and high retardation value. Since the cellulose acylate film also has a function of a protective film in the polarizing filter, the low-cost and thin liquid crystal display can be supplied in the market.
As already known methods for producing the polymer film, there are a melt-extrusion method in which the polymer is melt with heating and an extrusion thereof is made to obtain the film, and a solution casting method in which a dope containing the polymer, a solvent and the like is prepared and the casting of the dope is made to obtain the film. In a solution casting method, a dope is cast onto a support to form a casting film. Then the casting film is peeled as a film when having a self-supporting property. Thereafter the solvent within the film is vaporized by a drying process, and the film is wound up.
In the solution casting method, the peeled polymer film before wound up is applied various processes including the drying process and a process of cutting side edge portions of the film. While being applied these processes, the polymer film is supported and fed by rollers. As the rollers, there are drive rollers and non-drive rollers. The non-drive roller is used for determining a feeding path of the polymer film and increasing a stability of feeding.
The drive roller is used for transmitting the driving force to the polymer film so as to feed it downstream. As the drive roller, a suction roller is usually used. While feeding of the film, different film tensions may be required in the different processes, such as the casting process, the peeling process, the drying process and the winding process. In this case, the suction roller applies the driving force to the film so as to change the film tension. The suction roller has a plurality of suction holes on a contact surface thereof so as to suck the polymer film thereon while the feeding.
When the suction roller is used for feeding the film, since a complex power whose direction cannot identify acts on the film, the film is likely to be deformed. Further, the film can be deformed by the difference of the film tensions between upstream and downstream from the suction roller. In addition, when the polymer film slips, contracts or becomes deformed while contacting on edges of the suction holes, microscopic flaws are occurred on the film.
Recently, since the LCD becomes to have higher brightness, the flaws on the polymer film used as the protective film or the like are likely to be observed even if the flaw is microscopic. In case that the polymer film is used as the protective film in the polarizing filter of the LCD in which illumination light from a backlight passes through the polarizing filter, if there are microscopic flaws on the film, the flaws diffusely reflects the illumination light so that the quality of the displayed image is considerably worsen.
In considering this problem, Japanese Patent Laid-Open Publication No. 2002-194107 discloses a method for reducing occurrence of the flaws on the cellulose acylate film, in which the film is produced such that a difference between a surface temperature of a drive roller and the film temperature is small. Japanese Patent Laid-Open Publication No. 2003-094467 discloses a method for producing the film in which the suction roller has suction holes whose diameter is smaller than that of the conventional suction hole.
However, in the method of the Publication 2002-194107, although the occurrence of the flaws on the film can be reduced in a certain level, the drying process is not effectively performed because the drying temperature can be varied in a limited range and cannot be high. To dry the film, preferably the drying temperature is as well as high so as to vaporize the solvent within the film. In the method of the Publication 2003-094467, while the suction roller has no flaw on its surface, the occurrence of the flaws on the film can be reduced because the film is hardly deformed. However, when the flaws are occurred on the suction roller by long time use, the flaws on the roller cause the occurrence of the flaws on the film.
An object of the present invention is to provide a solution casting method for producing a cellulose acylate film, in which flaws on the film are hardly occurred, and a suction roller used therefor.